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If it wasn't for the arrival of plants on land 700 million years ago, you might not be reading this. In fact, humans might not have existed at all.

That's the conclusion reached by American researchers who have come up with a new date for the arrival of the first land plants - some 220 million years earlier than previously thought.

"Their presence could have had a profound effect on the climate and evolution of life on Earth," says Blair Hedges, leader of the research team. Their work is published in today's Science.

The scientists, from Penn State University, studied genetic sequences from a number of fungi, algae and vascular plants, and used the amount of genetic variation to calculate how long ago various groups diverged from each other.

They deduced that land plants evolved about 700 million years ago, and land fungi about 1300 million years ago. Previous estimates of around 480 million years ago were based on the earliest fossils of those organisms. But the researchers believe the primitive bodies of these early plants may have been simply too soft to be well preserved as fossils.

If their conclusions are correct, it's entirely feasible plants paved the way for the evolution of land animals - simultaneously increasing the percentage of oxygen in the Earth's atmosphere, and decreasing the percentage of carbon dioxide, a powerful greenhouse gas.

The earlier timing of the arrival of plants gives them a plausible role in the sudden appearance of many new species of fossil animals in an event known as the Cambrian Explosion, roughly 530 million years ago. It was during this time that almost all major categories of animals living today are first thought to have appeared on Earth.

"The plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger and diversify," Hedges says.

By absorbing carbon dioxide, the plants might also explain the "Snowball Earth" eras, when ice periodically covered the globe around 750 to 580 million years ago.

Hedges concedes other factors such as the location of the continents may have cooled the atmosphere to some extent.

"But I suspect the biggest cooling effect came from the reduction of carbon dioxide in the atmosphere by fungi and plants, which we have shown were living on the land at that time," he says.

Unlike previous studies of this kind, which analysed changes in a single gene, the team looked at 119 genes - a number they argue makes their conclusions more reliable.